cell shape and function.
- Importantly, the nucleus is compartmentalized. Here, separating the
cytoplasm from the nucleus, there is a nuclear membrane shown in the
slide, the nuclear envelope.
- Prophase is the first stage of mitosis and there are two main things that
start to happen, two main changes that occur.
- One is that sister chromatids start to condense, they align and they form
microtubule attachment sites. That is shown in the nucleus outlined in the
slide. Here in brown we see the DNA, these DNA double helices have been
replicated so now we have two copies of these so they were replicated
during interphase. Now we see these two pieces of DNA, each of them is
called a chromatid. The two chromatids are starting to connect together and
form one unit. Additionally, they form attachment sites for microtubules at
these red marks called kinetochore.
- At the same time this is happening, in the cytoplasm the centrosomes
begin to organize this bipolar microtubule spindle. Importantly, notice that
the microtubules and the DNA, they are still separated by a nuclear
envelope so they’re kept away from one another at this stage.
! Packaged and tagged to interact with microtubules
- Now to look at how these two steps happen.
- So we can think of this stage as preparing the DNA for mitosis and
preparing the microtubules for mitosis.
- For the DNA, there are basically 2 steps. One step is packaging them up
so instead of being very loose strands of DNA filling the nucleus, they are
packaged into solid packages that can be transported within the cell in an
effective way. So they’re packaged up and the other thing that is done to
them is that there is a tag put on them that is recognized by microtubules.
- There is basically 2 steps here, they’re packaged and tagged to interact
! Individually condensed by condensins
! Glued together by complexes called cohesins
- This packaging occurs when all the DNA is duplicated in S phase, there
are 2 copies of each chromosomes and these remain associated as sister
- During prophase, each chromatid is individually condensed by condensins
and that is shown in this picture here, this is one DNA coil, one of these
sister chromatids. This DNA in interphase would be very loose and filling
the cell but here, these condensins are basically coiling this up and
packaging it together sort of like how you would coil up a hose in your
backyard. It connects them together making them a tighter package.
- In this picture right here, we are looking at 2 different sister chromatids.
The chromatids remain attached here called the centromere so that is an
important part of the chromosomes there, where those 2 chromatids remain
attached. We can see that they are each individually condensed by these
condensins & here in pink we can see the condensins running along each
sister chromatid. They are separately compacted by these condensins. These
condensed sister chromatids are glued together by complexes called
- This cartoon up here, these are the two sister chromatids. These cohesins
are connecting these condensed chromatids together into one large package.
If we fit this into this cartoon right here, the cohesins would be running
down the center attaching one sister chromatid to the other sister chromatid
so they’re all one large unit.